Slug Flow Characteristics and Their Effect on Corrosion Rates in Horizontal Oil and Gas Pipelines

The flow characteristics of slug flow and their effect on the corrosion rates are examined experimentally in a 100 mm diameter, horizontal, Plexiglass pipeline. The system was maintained at a pressure of 2 bars and temperature of approximately 35 0C. Mter initial tests with oil only and water only, 80% water and 20% oil, and 60% water and 40% oil mixtures were examined. Carbon dioxide was used as the gas. Using the stationary slugs described by Jepson1,2, the mean wall shear stress and turbulent intensity at 7 different axial and three circumferential locations were measured using flush mounted TSI hot f1lm anemometer probes. The regions included the f1lm region ahead of the slug, the mixing zone at the slug front, and a short distance into the slug body. The oil, water, and carbon dioxide fractions were measured across a vertical diameter using. a sampling probe. Corrosion rates at the top and bottom of the pIpe were measured using electrical resistance probes. Pressure gradients across the slug front were also taken. The results clearly show that the slug front is a highly turbulent region that has large wall shear stresses at the bottom of the pipe. The wall shear stress decreases with circumferential position but increases with increasing oil fraction in the fluids. For a Froude number of 12, the shear stress for water only is 26 N/m2. For the mixture of 60% water and 40% oil a value of 97 N/m2 is attained. These are much greater than anticipated. Instantaneous values are much larger than this. The corrosion rates are much greater than those predicted by usual laboratory methods such as rotating cylinde~s or disks. The rates are shown to be grea;est at the bottom of the pIpe and correspond to the regions of high wall shear. Corrosion rates increase with an increase in oil concentration at the same Froude number. It is concluded that the wall shear stress associ~ted with the slug front is sufficient to continuously remove corrOSIonproducts from the pipe wall.